A. Field of the Invention
The present invention relates to the production of lightweight expanded silicate aggregates, and more particularly to a method of preparing a lightweight expanded silicate aggregate from rice hull ash.
B. Description of the Prior Art
The production of lightweight silicate materials by thermal expansion has been disclosed in the prior art. Such materials find use, among other things, as insulation and fire proofing material, aggregates used in the preparation of concrete, oil adsorbents, and additives for improving soil.
One method for producing a lightweight silicate aggregate is disclosed in Temple et al, U.S. Pat. No. 4,230,773. In general, the process of the Temple et al patent includes the steps of mixing finely divided anhydrous alkali metal silicate with a boron-containing curing agent and water until a homogenous, free flowing powdery composition is attained. The powdery composition is cured at a temperture below about 100.degree. C. until the silicate is hydrated. The hydrated composition is then comminuted and expanded by rapid heating to at least 425.degree. C. to produce the lightweight aggregate. The starting material, i.e. and anhydrous alkali metal silicate, is a highly refined, pure product.
Walls-Muycelo, U.S. Pat. No. 4,308,065 discloses another process for making particulate lightweight porous aggregates. The process of the Walls-Muycelo patent includes preparing an aqueous paste of an alkali metal hydroxide with a mineral containing aluminum oxide, silicone dioxide in mineral form, boric oxide, a volatile material, and an aggregate modifier. The aqueous paste is stirred and the temperature is allowed to be autogenously increased to a temperature of between 70.degree. C. and 80.degree. C. The stirring is discontinued when the temperature remains unaltered for a short period of time and the paste turns into a reaction mass. The reaction mass is dried at a temperature of about 300.degree. C. and then crushed and fired to produce the expanded aggregate.
Additional processes for preparing expanded lightweight silicate aggregates are disclosed in U.S. Pat. Nos. 3,719,510; 3,743,601; 3,765,919; 4,080,187; 3,756,839; 3,728,208; and 3,663,249. In the processes of all of the foregoing patents, the starting materials include pure alkali metal silicates.
Commercially, alkali metal silicates are manufactured by heating a mixture of an alkali compound and silica at about 2700.degree. F. until a technically homogeneous glass is formed, then cooling the alkali metal silicate glass ("water glass") or quenching and dissolving it in water. Alkali metal silicates for general commercial use are manufactured with ratios of SiO.sub.2 to R.sub.2 O(R=Na,K,Li,Rb,Cs) ranging from 1:1 to 3.75:1, while higher ratios up to 5:1 are less readily dissolved in water, hence of lesser commercial interest.
It is known that rice hulls contain silica. In Blardone, U.S. Pat. No. 1,293,008, there is disclosed a process for extracting silica from rice hulls. For example, Blardone discloses that a concentrated solution of sodium silicate may be extracted from rice hulls by boiling together for a period of from one-half hour to several hours a mixture of 4 or 5 parts by weight of burned rice hulls with 10 parts of water and 1 part of sodium hydroxide. After such period of boiling, the mixture is filtered, with the mass remaining in the filter being washed with water and the filtrate, which is a solution of sodium silicate, being concentrated to the desired degree by further boiling. Another process disclosed by the Blardone patent includes the steps of fusing together 3 or 4 parts of burned rice hulls and 1 part of sodium carbonate, or 8 or 10 parts of burned rice hulls and 3 parts of sodium sulfate. The fused product is then treated ith water to extract the sodium silicate, filtered, and the filtrate is evaporated to the required density.
Another patent, which makes use of the fact that rice hulls contain silica is Bieler et al U.S. Pat. No. 3,511,601, which discloses a process for densifying rice hulls. The process of the Bieler et al patent includes coating the rice hulls with an aqueous solution of an alkali metal hydroxide and then burning the coated rice hulls.
It is an object of the present invention to provide a lightweight silicate aggregate prepared from rice hulls without first forming substantially pure alkali metal silicate.